Shoaling fish. It sounds like the very pinnacle of social behaviour, every individual fish co-operating in perfect synchronisation for the benefit of the group but the truth is quite different.

Many fish shoal, predominantly open sea fish like Trout or Atlantic Salmon but also aquarium fish such as barbs, characins and danios, where as others for instance cichlids, do not. A shoal is a group of fish that remains together for social reasons, including feeding, breeding or resting and can consist of anything from a few fish to hundreds of thousands.

Before we begin let me clear up a two important definitions.

1. School. A group of fish that swim in a synchronised manner, i.e. with similar speeds and direction. They also display a consistent Nearest Neighbour Distance (NND), which means they maintain the same distance between all immediately adjacent fish.
2. Shoal. Fish which are randomly orientated within a group and exhibit a variable NND. Shoals of fish on the move nearly always form schools.

The choice to shoal is made by an individual fish. A continually changing environment means that fish constantly re-assess the choice to shoal and regularly leave or join the group only doing so if it benefits their current circumstances. Fish weigh up the advantages and disadvantages of shoaling, and only if benefits outweigh the costs will the shoal be maintained.

Most schooling fish maintain a NND of 0.5 to 1 times their body length. Remember that in large shoals only the fish at the front and sides of the group can see the "outside" world, all another fish can only see their neighbours. Yet, fast moving species still manage to perform rapid changes in direction to avoid predators or obstacles and remain in perfect synchronisation. How do they do this?

Shoals are maintained through the fish's senses. Primarily visual stimulus is utilised to recognise and relate to others. Fish look for markings, spots or stripes similar to their own. They also have the ability to identify colour and patterns. Subtle changes in the orientation or speed of colours and patterns on fish induce large changes in the reflections from them. These are detected by neighbouring fish allowing them to maintain their NND. Evidence of this effect can be seen in the home aquarium at night. It is thought that the lack of visibility at night is the reason why many shoaling fish disperse at dusk only to reform at dawn. If you keep small Tetras, look for individuals after lights out. They will be scattered throughout the tank hiding in plants or among rocks. The lack of light prevents the shoal from being effective, since the individuals cannot see so well.

Some fish utilise a different sense, the ability to detect pressure. It is believed that fish use sensors located in the lateral line to detect pressure waves produced as other fish swim by. The sensors react rapidly allowing quick transmission of information throughout the shoal ensuring synchronised swimming. It is the same detection of pressure by the lateral line that causes panic in fish if the aquarium glass is tapped.

So why do fish shoal? There are a number of reasons why a fish will or will not choose to shoal.

Primarily fish shoal for protection against predators. If there are two shoals one consisting of 1000 fish and the other consisting of 10 fish, then the odds of an individual being eaten in the larger shoal is 1 in 1000. In the smaller shoal, it is only one in 10. As the size of the shoal increases the probability for an individual being targeted reduces. Again we see here the benefit to the individual, not the group, for remaining in a shoal.

The shoal also has other advantages for predator avoidance. As mentioned earlier, fish have many senses to detect other members of the shoal but they also use these senses to detect predators. The number of fish in a shoal obviously increases the number of sensors working simultaneously to detect predators.

Appearance of a shoal can also be used to deter predators - the sheer numbers and synchronised movement of a shoal can confuse predators into thinking that the shoal is in fact one larger fish. Similarly, a synchronised twisting and turning of a shoal may simply bewilder an attacker. A large number of fish is also likely to present the predator with too many choices of which fish to attack allowing vital time for escape. However, evidence exists to suggest that shoaling may increase the chance of being detected by a predator. Many predators intimately associate with their prey, as is the case with Barracuda who often accompany shoals of Grunts, their natural prey. Shoaling also attracts aerial predators, as their size is more conspicuous from the air.

The success of the "safety in numbers" approach only works if the fish can communicate the detection of danger to other members of the shoal otherwise the shoal would not benefit form a co-operative defence.

The defence tactics used by shoals are also interesting. Predators of any species know that it is easier to single out and capture an individual rather than take on the group. It is therefore in the interest of the individual fish to remain in the shoal for all the reasons previously explained (reduced probability of capture, more warning systems at work, etc.).

During an attack, shoals communicate a number of defined escape manoeuvres. Some flee away and then U-turn past the attacker on either side, visually confusing him. Other species split into smaller shoals presenting the predator with a choice problem of which group to attack. Others employ both confusion and choice techniques as the shoal "explodes" in all directions. The drawback of these evasion methods is that if the shoal does not reform rapidly individuals are more likely to be picked off.

One other tactic, more characteristic of open sea fish, is to for the shoal to pack tightly together. This may be selfish ploy of the individuals attempting to hide behind other neighbours.

Interestingly, many predators that prey on shoaling fish also employ shoaling when capturing individuals. A simultaneous attack on a shoal by a group of predators is more likely to break up the shoal enabling individuals to be picked off. A simultaneous attack on a shoal should not be confused with a co-ordinated attack on a shoal. Where as many mammal predators, such as dolphins or even large cats, do co-operate in hunting with individuals carrying out specific roles in the attack, very few species of fish operate in this way. Yellow Tail Tuna, Thunnus albacares, may be the exception that proves the rule. These fish isolate groups of Mackerel and make forays into the group while other Tuna prevent a group escape.

Unfortunately, in a shoal resources are limited as there is invariably less to be shared among more fish for example, competition for food is greater. Scientists have found that generally, larger well-fed fish associate with larger shoals whereas hungry individuals associate with smaller shoals, as there is less competition for food. The larger shoals have predator avoidance as their number one priority. The smaller shoals forgo the security of a group to satisfy their feeding requirements.

However, food is unevenly distributed and difficult to find in murky waters so much of a fish's time is spent foraging. Just as many eyes allow better detection of a predator, it easy to see that in a group more fish foraging for food would produce a higher success rate in finding it. Analogous to way the detection of a predator is communicated through the shoal; the discovery of food is also announced. Scientists have proven these theories with Goldfish and discovered that randomly located food was located faster as the shoal size increased and that Goldfish adopt a head down position on the detection of food which attracts other individuals to the source. A fishes day is spent (ignoring reproduction) either feeding or avoiding predators. The less time they can spend looking for food, the more time can be used to look out for predators.

Shoaling is not only a benefit for feeding and predator avoidance. It is also believed that migrating fish shoal to conserve energy in a similar manner to flocking birds. Fish swimming in the slipstream of the fish in front are able to benefit from reduced drag effects of water. Another theory suggests that migrating fish such as Atlantic Salmon benefit from improved navigational accuracy within a shoal. Just as people have different navigational perceptions of direction and distance so do fish. It is thought that a shoal, with no defined leader, will effectively take a mean course from that of the group consensus to increase accuracy of migrations.

It is naturally though that only conspecific (fish of one species) fish will form a shoal however, anyone who has kept Cardinals and Neons together will know this is not the case. Multi-species shoals do occur in the wild and in most cases the benefits of predator avoidance are obvious but some fish also benefits from the feeding advantage especially if their diet differs from that of the predominant species. Given a choice though, fish prefer to shoal with their own species. Research as shown that even colour morphs of the same species choose to shoal with conspecifcs of their own colour, they need to associate like with like. There maybe a very valid reason for this in that being different for the crowd draws attention to the individual and thus leaves the fish prone to attack.

Back in the aquarium you may think that since you have provided a predator fee environment and food is readily available theoretically fish should not shoal. Unfortunately the fish does not know that the aquarium is predator free and will therefore anticipate an ambush from behind the next plant. Additionally, shoaling is a genetic feature of fish and thus they have a born urge to shoal. Aside from this, fishkeeping experience tells us that in order for a fish to feel happy and secure we must provide an environment similar to that found in their native habitat. (Remember most fish are wild creatures!) So apart form providing the right environmental factors we should accommodate the right psychological factors.